established procedure to document crop N status. These tests involve measuring organic N (also called
reduced N) in the leaves [28,29] or inorganic N (NO 3 ) in the stems [30,31] and can be used to determine
deficiencies as well as excessive applications of N. This technology, however, has typically been used in
diagnostic work rather than as a management tool because the measurements are usually made too late to
permit corrective N applications.
Leaf chlorophyll measurements have been advocated as a means of taking advantage of the close as-
sociation between chlorophyll and leaf N concentration to assess soil N availability and plant N status
[32,33]. The development of a handheld leaf chlorophyll meter (SPAD-502, Minolta Camera Co.) allows
rapid and nondestructive measurement of leaf greenness [34,35], and some evidence suggests that this
technique can be used as a management tool for making fertilizer N recommendations [36,37]. Other
work, however, has shown that widespread calibration of chlorophyll meters to determine crop N status
may not be practical, given differences in leaf greenness among cultivars and/or effects on the readings
of growth stage, N form, and management practices [32,38]. As a result, normalization procedures may
be necessary to standardize chlorophyll meter readings across cultivars, locations, and growth stages by
comparing readings from well-fertilized rows with those from the test area [32].
B. Nitrogen Accumulation
- Nitrogen Uptake
Plants acquire the vast bulk of their N from the soil via the root system. This process involves the move-
ment of inorganic N (NO 3 and NH 4 ) across membranes, transport or storage within the plant, and ulti-
mately assimilation into organic compounds. The uptake of both N forms is generally considered to re-
quire metabolic energy mediated by enzyme permeases located in or on the plasmalemma of external root
cells. Absorption of both forms is affected by the ion’s concentration in the external solution, with the up-
take rate exhibiting diminishing returns in response to increasing internal concentrations. Absorption is
also affected by external factors such as temperature and pH (see Section II.B.2).
The consequences to plant metabolism from the uptake of NO 3 and NH 4 are vastly different because
of differences in the charge of NO 3 and NH 4. With NH 4 nutrition, plants absorb cations in excess of an-
ions, resulting in a net efflux of Hfrom the root and an acidification of the external medium [9,10]. Con-
versely, with NO 3 nutrition, plants absorb an excess of anions, which causes the medium to become more
alkaline [9,10]. Also because of these differences in charge, the mechanisms for uptake by plant roots dif-
fer for NO 3 and NH 4.
In evaluations of N uptake, plants that have depleted their N supply (both in solution and in storage)
are typically used to observe all phases of uptake and the influence of N in inducing the uptake system.
For NO 3 -depleted plants, the pattern of NO 3 uptake generally exhibits a two-phase pattern, with an initial
lag period followed by an exponential increase in uptake [39–41]. The initial lag in NO 3 uptake is in con-
trast to that observed with many other ions [40,41] and suggests the induction of a specific NO 3 trans-
porter by NO 3. The accelerated phase of NO 3 uptake is also indicative of induction because it is depen-
dent on a critical NO 3 concentration in the root, in a manner similar to enzyme induction by its substrate
[41,42]. In addition, the accelerated phase is restricted by inhibitors of protein or RNA synthesis or by
conditions that limit or inhibit respiration [40,43]. Collectively, these studies show that the NO 3 uptake
system is dynamic and capable of adjusting to changes in the level of NO 3 in the root environment.
The uptake of NO 3 is an active process, which must overcome an unfavorable electrochemical gra-
dient between the soil and the root. However, because of this gradient, NO 3 can also efflux (or leak) back
out of the root. Efflux has been described as a passive diffusion process [44] or as a carrier-mediated pro-
cess [45] but in either case dependent on the internal concentration of NO 3 in the root. As a result, the net
accumulation of NO 3 is a function of the difference between influx and efflux. As might be expected, ef-
flux is greatest when high concentrations of NO 3 have been accumulated by root tissues [46,47].
Unlike NO 3 uptake, the absorption of NH 4 does not exhibit a prolonged lag under N-depleted con-
ditions [48], although uptake can also be characterized by two main phases [40]. The initial phase of
NH 4 is insensitive to low temperatures or metabolic inhibitors, hence is thought to occur passively
[40,49]. In contrast, the second phase of NH 4 uptake involves metabolic energy and is sensitive to low
temperatures and inhibitors [49]. In some plant species, the active phase of NH 4 uptake is also multi-
phasic, exhibiting uptake and growth rates associated with deficiency, luxury consumption, and toxic-
ity [40,50,51].
NITROGEN METABOLISM AND CROP PRODUCTIVITY 387